1. Field of the Invention
The present invention relates generally to a drivetrain for a vehicle. More particularly, the present invention relates to a vehicle drivetrain having a secondary clutch configured to interrupt transmission of torque through the drivetrain in response to torque loads above a threshold level that are applied to the drivetrain.
2. Description of the Related Art
In general, vehicles intended for off-road use employ a manually operated multi-speed transmission, which typically utilizes a manually controlled clutch assembly configured to interrupt the flow of torque through the vehicle's drivetrain. Such an arrangement provides the operator of the vehicle with direct control over the gear ratio selected for a particular situation. Conversely, off-road vehicles intended for younger, less experienced riders often utilize an automatically engaging clutch and single speed transmission.
Eliminating the need to shift gears and control engagement of a manual clutch reduces learning time for a younger, inexperienced rider. An automatically engaging clutch also allows the less experienced rider to fully concentrate on maneuvering the vehicle, rather than focusing on transmission gear selection and clutch operation. Furthermore, in certain competitive events for off-road vehicle (e.g., motocross or cross-country races), riders in certain age groups and/or displacement categories are required to compete aboard a single speed vehicle, which may also be equipped with an automatically engaging clutch.
A common automatically-engaging clutch is actuated by the engine speed of the vehicle. Such clutch mechanisms are often referred to as a centrifugal clutch. In such an arrangement, the clutch assembly is disengaged at engine speeds below a threshold engine speed, or an engagement speed. Once the engine reaches the engagement speed, the automatic clutch engages to transmit torque from the engine of the vehicle to the transmission of the vehicle, which drives one or more driven wheels of the vehicle.
Although an automatically-engaging clutch is advantageous in that it automatically engages and disengages the vehicle engine from the vehicle transmission without direct operator involvement, such an arrangement also suffers from disadvantages. For example, because the engagement force of the automatic clutch generally depends upon the engine speed, the clutch is unlikely to slip (permit relative movement between various components of the clutch) when engine speeds are high. As a result, sudden decelerations of the driven wheel(s) of the vehicle, when the engine speed is high, results in a large torque spike being transmitted through the vehicle drivetrain because the automatic clutch tends to remained engaged.
Such a situation is common when an off-road vehicle contacts the ground upon landing from a jump. Typically when landing from certain jumps, especially when the landing is on relatively flat ground, the vehicle operator will keep the engine speed high to ease the jolt of the landing and maintain momentum upon landing. As a result of high-torque loads, which may be significantly higher than the torque loads experienced during normal operation, certain transmission and engine components are typically over-built to withstand such loading characteristics. That is, the drivetrain is typically designed to accommodate loads that are relatively infrequent and often of short duration and which exceed the loads experienced the majority of the time during “normal” operation. As a result, such a drivetrain possesses an undesirable amount of inherent friction, and thus inefficiency, for most operating conditions.